Optical fibre transmission networks, and more particularly those of passive optical network type, are increasingly used to provide network access to residential or office gateways, or to ensure mobile front- and/or backhauling.
In an attempt to increase the number of users served through one access system to the network, wavelength division multiplexing technologies have been developed. These technologies take advantage of multiplexing several signals using different light carrier wavelengths in one optical fibre. Then, a wavelength splitter is used to select the signals supported by one light carrier and direct them to the intended user device. To this purpose, the wavelength splitter is placed on an optical fibre-path which connects the optical transmission device, for example an optical line termination, to the optical user device, for example an optical network unit. Several types of wavelength splitters currently exist, including thin films based devices, interference cavities as AWG (Array Wavelength Gratings) and FBG (Fiber Bragg Gratings). One wavelength splitter is commonly comprised of several filters which are combined with one another.
Generally, the wavelength splitter has two functions depending on the direction of the signals transmitted. For downstream transmission, i.e. from an optical line termination to an optical network unit, the wavelength splitter produces a signal selection based on the light carrier wavelength as previously mentioned, and forwards the signals selected through an output port of the wavelength splitter which is connected to the optical network unit. For upstream transmissions, i.e. from one among at least two optical network units to the optical line termination, the wavelength splitter collects the signals from the optical network units through respective optical fibres, each signal being supported by a dedicated light carrier, and combines them within one single optical fibre to the optical line termination. For performing these functions, the wavelength splitter has a filtering feature of passband type between one input port and each one of its output ports, which indicates the wavelength range of the signals—i.e. of the light carrier of each signal—that can be transferred from the input port of the wavelength splitter which is connected to the optical line termination, to the output port of concern which is connected to at least one optical network unit.
But the wavelength splitter may be located in any environment between the optical line termination and the optical network units, for example along a street or in the country, and is subject to environment parameter variations such as temperature variations. Then, the wavelength splitter may be capable to transfer the signals supported by one light carrier downstream from the optical line termination to the optical network unit during a time period, and may be no longer capable of transferring signals supported by the same light carrier, from the same optical line termination to the same optical network unit, after one environment parameter has varied in an uncontrolled manner. Such operation failure is called detuning by the Man skilled in the art, thereby indicating that uncontrolled variations at the wavelength splitter has caused a mismatch between the filtering feature of the wavelength splitter and the light carrier wavelength implemented by the optical line termination. Detuning also affects the upstream signals in a similar manner. For both downstream and upstream signals, detuning decreases the transmission quality of the optical fibre transmission network, and can even inhibit the transmission capacity.
It is possible to ensure continuing transmission at the wavelength splitter whatever the variations of the environment parameters, by providing stabilization of these parameters as they affect the wavelength splitter. For example, temperature at the wavelength splitter may be controlled, possibly by using a Peltier element. However, any system designed for compensating for the variations of at least one environment parameter is power-consuming, and therefore must be supplied with energy, most often with electrical power, from a power source. But implementing such energy supply may be complicated and very expensive for some locations of the wavelength splitter, for example when it is far from inhabited areas. In addition, because the optical fibre transmission network may involve a great number of wavelength splitters, it is of main interest that each one or at least some of these wavelength splitters can be installed in a simple and cost-effective manner.
Then, starting from the requirement that the wavelength splitters should not be combined with environment variation compensating systems, passive optical networks have been proposed. In these passive optical networks, the wavelength of a light carrier is adapted to the currently existing filtering feature of the wavelength splitter. Put another way, the effect of the uncontrolled environment parameters onto the wavelength splitter are taken into account by the source of the light carrier, so that the carrier wavelength keeps matching the filtering feature of the wavelength splitter. No power source is necessary any longer at the wavelength splitter. But then, a challenge consists in detecting a detuning which may appear between the light carrier as currently produced by the optical line termination and the actual filtering feature of the wavelength splitter.
An additional issue is then to compensate for such detuning for restoring maximum transmission capacity through the wavelength splitter.
In other respects, it is known to produce several light sub-carriers from one and same light carrier, so as to implement orthogonal frequency-division multiplexing. Such sub-carriers may be produced by modulating the light carrier simultaneously with several hyper-frequency components, these components being further phase- and/or amplitude-modulated for encoding the data to be transmitted. At a wavelength splitter, all the sub-carriers are transmitted between one same optical input port and one same optical output port of the wavelength splitter, since the differences in wavelength between any two sub-carriers are far less than the wavelength difference which exists between two light carriers.
Starting from this situation, one object of the present invention is to allow detecting a detuning which may exist at a wavelength splitter, in an efficient manner when optical sub-carriers are implemented in the optical fibre transmission network.
An additional object of the invention is to allow such detuning detection in an optical fibre transmission network of passive optical network type.
Still another object of the invention is to allow compensation for a currently existing detuning, while avoiding that the data transmission capacity is significantly impaired.